Patients with refractory metastatic cancer will receive 5-Ethynyluracil (776C85) (an inhibitor of uracil reductase) alone or in combination with 5-fluroruracil/leukovorin to determine the safety/efficacy of 776C85 and its effect on the pharmacokinetics of 5-FU. 776C85 has no antitumor activity of its own but, by reducing the clearance of 5-FU, may decrease pharmacologic variability and/or improve the efficacy of 5-FU.